1. Field of the Invention
The invention is directed in general to electrolytes of solid oxide fuel cells, and more particularly, to an improved electrolyte structure and associated method of manufacturing same.
2. Background Art
Solid oxide fuel cells (SOFCs) are well known in the art. Indeed, SOFCs are recognized as having the potential to mitigate environmental problems while meeting the power generation and cogeneration needs of tomorrow. Thus, much emphasis has been concentrated on the lowering of the SOFC operating temperatures while increasing SOFC stack performance.
In particular, one such development has been the use of the electrode supported thin film electrolytes for high performance SOFC systems. Processes for manufacturing thin electrolyte films include Allied Signal roll calandering, Lawrence Berkeley Labs and University of Missouri (Rolla) spin coated sol-jel, University of Utah dip coated, Dow tape laminate, and Westinghouse electrochemical vapor deposition (EVD) processes. Each of these processes are capable of producing 5-50 .mu.m electrolyte layers on an electrode substrate. In particular, calendering and tape lamination processes appear to be the most promising and have demonstrated some levels of commercial viability in electronic chip packaging.
While these types of structures have been demonstrated to be quite successful in small area single cells, these structures have not been applicable with success to stackable large area cells. In particular, thin film electrolyte relies on the substrate electrode material for mechanical support. The mechanical, chemical and micro structural requirements of the electrode function are incompatible with the mechanical support functions of the electrode. In addition, due to the porous microstructure required by the electrode, the mechanical strength of the electrode is quite poor. With both the anode and the cathode materials, volume and composition changes accompany oxygen potential changes, and, these changes all greatly affect the mechanical integrity of the cell.
In addition, the exposure to the oxygen potential gradients are unavoidable in the prior art inasmuch as the supporting electrodes extend to cell edges so as to fully support the thin film electrolyte, exposing cathodes to fuel and anodes to air. In addition, the use of nickel cermet anode substrates which are fired in air as oxide, is likewise detrimental to the mechanical integrity of the cell. In particular, as the NiO is reduced to nickel as the cell begins operation, the cell is detrimentally affected relative to its mechanical strength.
One solution has been disclosed in Ishida, U.S. Pat. No. 5,312,700. This reference contemplates the use of intersecting support ribs on a single plane, on one or both sides of a thin plate for increased support and rigidity of the electrolyte. While this disclosure addresses certain of the shortcomings relative to strength, the intersecting support structure is difficult, time consuming and costly to manufacture.